1. Field of the Invention
The present invention relates to a high frequency heating method of a bolt with hole for heating the bolt with hole for fastening and loosening thereof by inserting a coil into a central hole of the bolt with hole used for fastening a steam turbine casing and the like and by supplying a high frequency current to the coil.
2. Description of the Prior Art
A casing of a steam turbine and the like consists of two split bodies of an upper case and a lower case, which are fastened together to form one unit by a bolt inserted into a hole bored in a flange of the upper case and the lower case, respectively.
In an operation condition, the bolt is fastened firmly to the casing for securing "gas tightness of the casing", but in case of opening the casing for inspection, maintenance services, etc. of the turbine, the bolt is to be loosened for separation of the upper case and the lower case.
For such cases of fastening the bolt firmly to the flange for securing gas tightness of the casing or of loosening the bolt for opening the casing, heating of the bolt is carried out for elongation thereof.
For example, in the Japanese laid-open patent application No. Hei 7(1995)-114984, a bolt heating means by use of a coil is disclosed wherein the coil is inserted into a central hole of the bolt and a high frequency current is supplied to the coil so as to generate an eddy current in an inner surface of the central hole and to heat the inner surface of the central hole by Joule's heat caused by an inside resistance against the eddy current, thus the heat is transmitted toward an outer surface of the bolt so that the bolt is heated in its entirety to be elongated.
A high frequency heating method of said kind of bolt with hole is described further with reference to FIGS. 6 and 7 by use of actual means according to the method mentioned above.
In FIG. 6, in case of loosening a metal bolt 22 which is set to a flange 21, a heater 20 is first inserted into a small diameter hole 22a of the metal bolt 22 to come to a position of a stopper 13 and a load dispatching portion 23 is connected to a high frequency transformer 24.
Then, as shown in FIG. 7(c), a constant high frequency power is supplied continuously to a coil of the heater 20 from the high frequency transformer 24 via the load dispatching portion 23 and a flexible cable 28 so as to generate an eddy current in an inner surface of the metal bolt 22 and thereby to heat the metal bolt 22 to be elongated and loosened. Then, a nut 25 is removed.
It is to be noted that said high frequency heating method is also applicable to a case where the metal bolt 22 is to be fastened, and in this case, the metal bolt 22 is first set to the flange 21 and then the same high frequency heating as mentioned above is carried out and the nut 25 is fastened.
According to said prior art method using the high frequency coil, while the bolt can be heated quickly, the inner surface of the central hole of the bolt is heated by Joule's heat of the eddy current and the heat is transmitted to the outer surface of the bolt to heat the entire bolt, thus there occurs a temperature gradient within the bolt.
And if this temperature gradient become large, a thermal stress with the bolt becomes large and there arises a problem to deteriorate a material of the bolt.
Accordingly, in case the bolt is heated by use of the high frequency coil as mentioned above, it is desirable to heat the bolt as quickly as possible within the range not to deteriorate the material of the bolt by the internal thermal stress.
Also, according to the prior art high frequency heating method of the bolt with hole described with the use of the actual means, a large power can be supplied so as to loosen the bolt within a short time as the result of employing such construction as described there.
However, in case the surface of the small diameter hole of the bolt is exothermically heated in the process of heat input, there may be caused an excessive temperature difference distribution .DELTA.T within cross sections of the bolt as shown in FIG. 7(a) and in this case, a compression stress .sigma. which acts on the surface of the small diameter hole of the bolt exceeds a yield stress .sigma.ys as shown in FIG. 7(b), hence fatigue damages are accumulated on the surface of the small diameter hole and there may occur cracks of the bolt after a long time use.
Also, as the temperature of the surface of the small diameter hole rises quickly, if a large power is supplied continuously, the temperature exceeds a final heat treated temperature To of the bolt material as shown in FIG. 7(a) resulting in lowering of the yield stress or a creep rupture strength of the bolt so that there may be a loosening or a crack damage of the bolt during operation.